This invention relates to power generating systems such as alternators and power generators, and more particularly to an alternator/inverter having a dual H-bridge for providing 120 volt AC or 240 volt AC power.
Present day portable generators typically make use of a synchronous alternator or a cycloconverter for providing the desired power output, which is typically either 120 volts AC or 240 volts AC. Important considerations for any portable generator are:
Voltage regulation;
Dual voltage output capability;
Idle voltage and frequency;
Frequency tolerance;
Harmonic distortion:
Induction motor operation
Charger operation
Grounding configuration;
4-blade (120-240 volt) twist-lock compatibility;
Response to load changes; and
Size and weight.
With regard to idle voltage and frequency, it is far easier to provide 120 volts and 60 Hz at idle using electronic solutions (i.e., inverter technology) than it is with synchronous alternators. However, sufficient xe2x80x9chead roomxe2x80x9d is still required. To provide 120 volts at 2100 rpm requires a DC bus voltage of 298 volts versus the 225 volts estimated for regulation head room. This higher voltage requires more turns in the alternator coils resulting in an increased coil resistance and reduced system efficiency.
Harmonic distortion present in the output waveform of a portable generator is another important consideration that must be addressed. While waveform purity is of little importance to universal motor-powered portable power tools, it is an important consideration when running induction motors and chargers. Induction motors will run on distorted waveforms, but the harmonic content of the input will be converted to heat, not torque. The extra heating from the harmonics must be quantified if a inverter topology which produces a distorted waveform is to be implemented. A sine wave pulse width modulated (PWM) inverter will produce excellent waveforms with only some high frequency noise, but they are likely to require full H-bridges which, traditionally, have not been easily adaptable to the North American grounding convention and the 4-blade twist-lock wiring convention.
With regard to grounding configurations, in North America, the standard grounding convention requires that one side (neutral) of each 120 volt circuit is grounded. This means that 240 volt circuits have floating grounds. It is difficult to achieve this standard grounding convention with sine wave PWM inverters that require full H-bridges. It is possible to meet this convention through the use of two half bridges, but such a circuit may be limited to quasi-sine wave outputs which have high harmonic content.
Still another important consideration is 4-blade (120-240 volt) twist lock compatibility. This convention requires four wires: ground, neutral, 120 volt line 1 and 120 volt line 2. Each 120 volt circuit is connected between a 120 volt line and neutral. The 240 volt circuit is connected between the 120 volt line 1 and the 120 volt line 2. Heretofore, it has been possible to fit the convention with a dual half bridge circuit, but not a full H-bridge circuit that would be required for sine wave PWM inverters.
The ability of a generator to respond to load changes is still another important consideration. All inverter topologies will provide a faster response to load changes than a synchronous alternator, due to the large field inductance used by a synchronous alternator.
Concerning size and weight, it would also be desirable to make use of inverter topology because virtually any inverter topology will provide size and weight benefits over that of a synchronous alternator. However, trying to produce sine waves from a two half bridge circuit may require large capacitors that would reduce the benefit of volume reduction provided by the inverter topology.
In view of the foregoing, it is a principal object of the present invention to provide a generator which meets the grounding convention used in North America through the use of inverter technology. It is still a further object of the present invention to provide a generator using inverter technology which can provide either 120 volt or 240 volt outputs and still meet the grounding convention used in North America.
Still further, it is an object of the present invention to provide a generator using inverter topology which meets the 4-blade twist-lock compatibility requirements.
The above and other objects are provided by an alternator/inverter system having dual alternator/inverter sections, with each section including a full H-bridge inverter circuit. Each alternator/inverter section incorporates an independent permanent magnet generator winding which is coupled to an independent full wave bridge rectifier circuit. Each rectifier circuit provides a DC voltage to its associated full H-bridge circuit. The first H-bridge circuit includes a first output point and a second output point while the second H-bridge circuit includes a third output point and a fourth output point. The second and third output points are coupled together as a neutral node and connected to ground. A first AC receptacle is coupled across the first output point and neutral. A second AC receptacle is also coupled across neutral and the fourth output point. A third AC receptacle is coupled across the first and fourth output points. Coupled across the third AC receptacle is an electronically controlled switch for selectively shorting the third AC receptacle. The switch is controlled by an electronic controller which also controls operation of each of the H-bridge circuits. A user switch allows a user to select a first mode of operation wherein full power developed by the alternator/inverter system may be drawn from either of the first or second AC receptacles, or a second mode of operation in which the third AC receptacle is operable. In one preferred embodiment, each of the first and second AC receptacles provide 120 volts AC, and the third AC receptacle provides 240 volts AC. In the second mode of operation, only half the total ampere generating capacity of the system is available at either of the first and second 120 AC receptacles as compared to that which would be available if the alternator/inverter system was operating in the first mode of operation. Importantly, the present invention adheres to the grounding convention used in North America in which one leg of each of the first and second AC receptacles is tied to ground.
The alternator/inverter of the present invention further provides excellent control over the harmonic distortion of the output waveform. The use of inverters allows a faster response to load changes than what would be obtainable with a synchronous alternator.
In alternative preferred embodiments, both analog and digital voltage regulation circuits are incorporated into the alternator/inverter system of the present invention. The voltage regulation circuits are employed together with active rectifiers to control the DC bus voltage across each inverter between predetermined upper and lower limits. In this manner, losses associated with electrical cabling coupled to the outputs of the two inverters, as well as losses associated with the inverters themselves, can be compensated for.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.